As is known, germanium oxide (GeO.sub.2) is chiefly used as dopant for fabricating an optical-fiber core, both in case of inside processes (IVPO) and in case of outside processes (OVPO). In fact, GeO.sub.2 gives with silica a binary compound having a stable vitreous structure. Besides, a haloid vehicle, wherefrom germanium tetrachloride (GeCl.sub.4) is obtained by oxidation synthesis, is particularly well suited for use in CVD techniques, since, at room temperature, it is an easily vaporizable liquid (melting temperature T.sub.F =49.5.degree. C.; boiling temperature T.sub.E =84.degree. C.).
The optical properties of germanium oxide are particularly interesting: zero material dispersion of wavelengths greater than 18 .mu.m, infrared absorption peak due to molecular vibration of G.sub.e -O bond centered at a wavelength of about 12 .mu.m.
The latter property prevents it from modifying the silica spectral-attenuation curve, which presents an infrared absorption peak for the molecular vibration of Si-O bond centered at a slightly lower wavelength (9.1 .mu.m).
For these reasons germanium oxide is nowadays the most widely used dopant compound in optical-fiber technology and the only one used for fabricating the core of silica-based optical fibers.
Its use has, however, two disadvantages:
(i) high cost of raw material; and PA0 (ii) Rayleigh scattering coefficient higher than that of pure silica, whose value is about 0.6 dB/Km/.mu.m.sup.4. PA0 (a) Rayleigh scattering coefficient less than to that of silica; PA0 (b) lower cost of raw materials; and PA0 (c) high melting temperature.
Germanium in the lattice structure of binary SiO.sub.2 -GeO.sub.2 is capable of increasing the scattering coefficient value proportionally to the concentration of the dopant present in the lattice;
For example, in the case of 3% molar germanium concentration (typical concentration for a monomode step-index fiber with .DELTA.n=3%, optimized for the second transmission window at 1.3 .mu.m) the Rayleigh scattering coefficient undergoes an increase of 0.2 dB/Km/.mu.m.sup.4. If the molar concentration is increased to beyond 20% to obtain the displacement of the zone of minimum chromatic dispersion, a value of Rayleigh scattering coefficient higher than 2 dB/Km/.mu.m.sup.4 would be reached. That is detrimental to performance because of the excessively high rise of minimum attenuation values.
Alumina (Al.sub.2 O.sub.3) is an alternative material to GeO.sub.2, in fact, in addition to having all the advantages of germanium oxide, it affords the following characteristics:
It is of interest to underline the fact that a scattering coefficient lower than that of silica can allow the lowest attenuation levels to be reached for silica-based vitreous lattices.
More particularly with vitreous lattices of SiO.sub.2 -Al.sub.2 O.sub.3 a minimum attenuation value lower than that of silica can be obtained; for silica this value is equal to 0.12 dB/Km in the wavelength range of 1.56 .mu.m.
Point (c), i.e. high melting temperature, allows a number of interesting remarks. The melting temperature of alumina (2045.degree. C.) is higher than those of silica (1703.degree. C.) and of germanium oxide (1086.degree. C.).
The physical properties of the lattices SiO.sub.2 -Al.sub.2 O.sub.3 are hence more similar to those of an SiO.sub.2, lattice than those of an SiO.sub.2 -GeO.sub.2.
In addition, the presence of a compound with higher melting point prevents the dopant from diffusing towards the periphery during the preform collapsing step.
As a consequence, alumina-doped silica fibers fabricated by the MCVD technique do not show any dip (i.e. central refractive index decrease). This is a typical anomaly in the profile of germanium-oxide doped silica fibers, fabricated with the same method.
A confirmation of the latter property has been already reported in the paper entitled "Fabrication of Low-Loss Al.sub.2 O.sub.3 doped silica fibers" by Y. Ohmori et al, Electronics Letters, Sept. 2, 1982, Vol. 18, No. 18.
The main disadvantage preventing alumina from being industrially utilized is that liquid or gaseous compounds at room temperature, to be used as aluminum vehicles and hence suited to CVD techniques, do not exist.
Aluminum halides are solid at room temperature and have rather high boiling temperatures. For example, AlF.sub.3 sublimes at 1291.degree. C., AlCl.sub.3 sublimes at 178.degree. C., AlBr.sub.3 melts at 97.degree. C. and boils at 263.degree. C., AlI.sub.3 melts at 191.degree. C. and boils at 360.degree. C. The use of the CVD technique with such raw materials requires reactant mixing and vaporization lines thermostated at high temperature. That entails difficulties and does not assure pollution-free synthesis products.
Besides, solid compounds at room temperature are more difficult to purify than liquid or gaseous ones; hence they can contain residual impurities detrimental to optical properties.
The use of AlCl.sub.3 as a basic aluminum vehicle has been already suggested in the above cited paper, yet no valuable result has been obtained.